Casey's most famous project was the completion of the Washington Monument.[1] Construction on the monument had been abandoned 24 years earlier, in 1854, due to technical and managerial problems. In its place stood a 170-foot tall pile of marble which was leaning to the northwest due to an unstable foundation. The structure was a public embarrassment and target of criticism from newspapers across the nation.[1]

Casey was hired to finish the job in 1878.[1] Casey, then a lieutenant colonel and head of the Office of Public Buildings and Grounds in Washington, D.C., was known for his engineering skills and financial trustworthiness.[1] He developed an ingenious method to balance the foundation as workmen with picks shovels dug from either side in a coordinated effort. Eventually Casey's men replaced half the old foundation, making it 13 feet deeper and over twice as wide.[1]

Over 130 years later, after the monument was damaged by an earthquake in 2011, engineers tasked with repairing the structure consulted Casey's original papers to understand how it was built. The 21st Century engineers called Casey's work "brilliant."[1]